Method for determining phase angle in phase shift transformer for medium voltage inverter

ABSTRACT

A method for determining phase angle in phase shift transformer for medium voltage inverter is disclosed, the method including selecting an arbitrary phase shift angle and phase angle relative to a unit power cell of the first stage, determining a phase angle displacement in consideration of the phase shift angle and the number of unit power cells connected to each phase of a motor, determining a phase angle of the unit power cell at the second stage using a phase angle of the unit power cell at the first stage, and adjusting a phase angle when the determined phase angle of the unit power cell at the second stage exceeds a predetermined phase angle.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C.§119 (a), this application claims the benefit ofearlier filing date and right of priority to Korean Patent ApplicationNo. 10-2013-0094081, filed on Aug. 8, 2013, the contents of which areall hereby incorporated by reference in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Field

The teachings in accordance with the exemplary embodiments of thispresent disclosure generally relate to a method for determining phaseangle in phase shift transformer for medium voltage inverter.

2. Background

In general, a multilevel medium voltage inverter is an inverter havingan input power whose rms (root mean square) value is over 600V for aline-to-line voltage, and has several stages in output phase voltage.The multilevel medium voltage inverter is generally used to drive anindustrial load of large inertia ranging from several kW to several MWcapacities of, for a non-limiting example, fans, pumps, compressors,tractions, hoists and conveyors.

The multilevel medium voltage inverter uses a phase shift transformer toreduce harmonics, where a phased shift angle of the phase shifttransformer is determined by the number of unit power cells, and anincreased number of unit power cells improve a THD (Total HarmonicDistortion) at an input terminal. However, if the number of unit powercells disadvantageously generate the THD at an input phase current.

FIG. 1 is a circuit diagram illustrating a configuration of aconventional multilevel medium voltage inverter, which is a schematicview illustrating a serially cascaded H-bridge multilevel inverter, andFIG. 2 is a schematic view illustrating a detailed configuration of unitpower cells of FIG. 1.

A phase shift transformer (110) in a general multilevel medium inverter(100) changes phase and size of voltage in a high input power inresponse to requirement of a unit power cell (120). An output voltage ofthe phase shift transformer (110) is an input power of each unit powercell (120), and converted to a DC through a 3-phase diode rectifier(121).

FIGS. 3 a and 3 b illustrate a structure and a phase diagram of a phaseshift transformer (110) where a phase shift angle at a secondary side ispositive to a phase shift angle at a primary side (Y/Z-1), when theprimary side of the phase shift transformer (110) is formed in a Ywinding of N₁ turn, a secondary side is formed in a delta (Δ) winding ofN₂ winding, and a tertiary side is formed in a winding of N₃.

Furthermore, FIGS. 4 a and 4 b illustrate a structure and a phasediagram of a phase shift transformer (110) where a phase shift angle ata secondary side is negative to a phase shift angle at a primary side(Y/Z-2), when the primary side of the phase shift transformer (110) isformed in a Y winding of N₁ turn, a secondary side is formed in a delta(Δ) winding of N₂ winding, and a tertiary side is formed in a winding ofN₃.

As noted from the foregoing, an phase shift angle of the phase shifttransformer (110) is determined by the number of unit power cells (120),where Ax, Bx, Cx of the unit power cells (120) respectively have a samephase shift angle. An output of secondary side of the phase shifttransformer corresponds to the number of the diode rectifier (121) atthe unit power cell (120), and a phase shift angle of the phase shifttransformer (110) may be determined by the following Equation.

$\alpha_{\sec} = \frac{360}{2N_{\sec}}$

where, unit of α_(sec) is i [degree], and N_(sec) is the number ofoutputs at the secondary side of the phase shift transformer (110), or atotal number of unit power cells (120). For example, N_(sec) is 9, andα_(sec) is 20 in FIG. 1. Thus, an entire phase shift angle may beselected as 0°, 20°, −20° based on 0°.

When two unit power cells are used for each phase of a motor, N_(sec) is6 and α_(sec) is 30°. In this case, a phase shift angle may be selectedas 0°, 30° based on 0°.

The conventional phased shift transformer thus described can outputthree pairs of secondary winding having a same phase shift angle, suchthat a problem of decreased THD in input phase current at the power sidearises when the number of unit power cells for each phase of a motor isfewer than three.

This problem is caused by disability in selection of sufficiently smallsize of phase shift angle in the phase shift transformer (110), suchthat the conventional structure of phase shift transformer (110)disadvantageously generates a problem of satisfying the THD harmonicsregulation at an input phase current in the system, only when more thanthree unit power cells for each phase of a motor are connected.

SUMMARY OF THE DISCLOSURE

The present disclosure is to provide a method for determining phaseangle in phase shift transformer for medium voltage inverter configuredto mitigate a THD of input phase current at a primary side of a phaseshift transformer, even when the number of unit power cells connected toeach phase of a motor of a multilevel medium voltage inverter is fewerthan three.

In one general aspect of the present disclosure, there is provided amethod for determining phase angle in phase shift transformer for mediumvoltage inverter, the inverter including a plurality of unit power cellsformed in first and second stages, one stage formed with 3-phase unitpower cells, and the plurality of unit power cells respectivelyconnected to a phase shift transformer, the method comprising:

selecting an arbitrary phase shift angle and phase angle relative to aunit power cell of the first stage;

determining a phase angle displacement in consideration of the phaseshift angle and the number of unit power cells connected to each phaseof a motor;

determining a phase angle of the unit power cell at the second stageusing a phase angle of the unit power cell at the first stage; and

adjusting a phase angle when the determined phase angle of the unitpower cell at the second stage exceeds a predetermined phase angle.

Preferably, but not necessarily, the predetermined phase angle may be30° at the maximum.

Preferably, but not necessarily, the adjustment of the phase angle maybe performed by using the following Equation.

$,{{X^{\prime}n} = {{{sgn}({Xn})}\frac{{6{{Xn}}} - 360}{6}}},$

where X′n is an adjusted phase angle of the unit power cell, Xn is aphase angle of the unit power cell (n is positive integer), and sgn(Xn)is a function determining a sign, where when

$\frac{{6{{Xn}}} - 360}{6}$is positive, ‘1’ is outputted and when

$\frac{{6{{Xn}}} - 360}{6}$is negative ‘−1’ is outputted.

Preferably, but not necessarily, the phase angle displacement may be avalue in which the phase shift angle of the unit power cells in thefirst stage is divided by the number of unit power cells connected toeach phase of the motor.

Preferably, but not necessarily, the phase angle of the plurality ofunit power cells may be individually determined relative to each unitpower cell.

Preferably, but not necessarily, the phase angle of unit power cells atthe second stage may be a value in which the phase angle of unit powercells at the first stage is added by the phase angle displacement.

Preferably, but not necessarily, the unit power cell of a tertiary stagemay be further connected to the phase shift transformer, and the phaseangle of the unit power cell at the tertiary stage may be a value inwhich the phase angle of the unit power cell at the first stage is twiceadded by the phase angle displacement.

Advantageous Effects of the Disclosure

The present disclosure has an advantageous effect in that a THD of inputphase current at a primary side of a phase shift transformer can bemitigated, even when there is a fewer number of unit power cellsconnected to each phase of a motor in a multilevel medium voltageinverter.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram illustrating a configuration of aconventional multilevel medium voltage inverter.

FIG. 2 is a schematic view illustrating a detailed configuration of unitpower cells of FIG. 1.

FIG. 3 a is a structural view illustrating a phase shift transformer ofFIG. 1.

FIG. 3 b is a phase diagram illustrating a phase shift transformer ofFIG. 1.

FIG. 4 a is a structural view illustrating a phase shift transformer ofFIG. 1.

FIG. 4 b is a phase diagram illustrating a phase shift transformer ofFIG. 1.

FIG. 5 is a circuit diagram illustrating a configuration of a multilevelmedium voltage inverter according to the present disclosure.

FIGS. 6 and 7 are respectively schematic views illustrating a detailedconfiguration of unit power cells of FIG. 5.

FIG. 8 is a flowchart illustrating a method for a phase angle accordingto an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Various exemplary embodiments will be described more fully hereinafterwith reference to the accompanying drawings, in which some exemplaryembodiments are shown. The present inventive concept may, however, beembodied in many different forms and should not be construed as limitedto the example embodiments set forth herein. Rather, the describedaspect is intended to embrace all such alterations, modifications, andvariations that fall within the scope and novel idea of the presentdisclosure.

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 5 is a circuit diagram illustrating a configuration of a multilevelmedium voltage inverter according to the present disclosure, where FIG.5 illustrates the inverter formed with unit power cells of two stagesfor each phase in a motor (30).

Referring to FIG. 5, the multilevel medium voltage inverter (10)includes a phase shift transformer (11) and a plurality of unit powercells (12).

The multilevel medium inverter (10) according to the present disclosuresupplies a 3-phase power to the motor (30) from an input power (20). Themotor (30) is a high voltage 3-phase motor and may be an induction motoror a synchronous motor, but is not limited thereto.

The phase shift transformer (11) provides a galvanic isolation betweenthe input power (20) and the multilevel medium voltage inverter (10),mitigates a harmonic of an input terminal, and provides an approximate3-phase power to each unit power cell (12). A phase shift angle of thephase shift transformer (11) is determined by the number of unit powercells (12), and phase shift angles of the phase shift transformer (11)connected to each unit power cell (12) have different values from eachother.

The unit power cell (12) receives a power from the phase shifttransformer (11) to output a phase voltage of the motor (30), where eachunit power cell (12) is comprised of three groups. As in the example ofFIG. 5, A1 and A2 are serially connected to synthesize ‘a’ phase voltageof the motor (30), and B1 and B are serially connected to synthesize ‘b’phase voltage of the motor. Furthermore C1 and C2 are serially connectedto synthesize ‘c’ phase voltage.

The synthesized ‘b’ phase and ‘a’ phase voltages are mutually apart witha 120° phase difference, the synthesized ‘c’ phase and ‘b’ phasevoltages are also mutually apart with a 120° phase difference.

FIGS. 6 and 7 are respectively schematic views illustrating a detailedconfiguration of unit power cells of FIG. 5, where FIG. 6 is an exampleof an inverter unit (63) in full-bridge inverter configuration, and FIG.7 is an example of an inverter unit (73) in a single phase NPC (NeutralPoint Clamped) configuration.

Referring to FIGS. 6 and 7, rectifiers (61, 71) rectify a 3-phase powerinputted from the phase shift transformer (11), and DC terminalcapacitors (62, 72) smooth the rectified 3-phase power. The full-bridgeinverter unit (63) may be configured with a 5-level unit power cell, andthe single phase NPC inverter unit (73) may be also configured with a5-level unit power cell. However, it should be apparent to the skilledin the art that configuration is not limited to the present disclosure,and therefore detailed explanation is omitted herefrom.

Determination of phase angle of the phase shift transformer (11)according to the present disclosure is performed as the method of thepresent invention at the time of design. Now, the determination of phaseangle in the phase shift transformer will be described with reference todrawings.

FIG. 8 is a flowchart illustrating a method for a phase angle accordingto an exemplary embodiment of the present disclosure.

Referring to FIG. 8, the method of the present disclosure is such thatphase angles of A1, B1 and C1, which are unit power cells (12) of firststage, are arbitrarily selected (S1). At this time, a phase shift anglemay be selected as 20°, and phase angles of A1, B1 and C1 may beselected as 0°, 20° and −20°. However, the given phase shift angles areonly provided as examples, and are not limited thereto, and orders ofphase angles of A1, B1 and C1 may be also changed.

Next, displacement of phase angle is determined (S2). The displacementof phase angle may be determined by the following equation inconsideration of the number of unit power cells connected to each phaseof a motor.

$\begin{matrix}{{\Delta\alpha}_{\sec} = \frac{20}{N_{sec\_ phase}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$where,

Nsec_phaseis the number of unit power cells (12) connected for each phase of themotor (30). Referring again to FIG. 5, Nsec_phase is 2, and therefore

Δαsec10^(∘),from which phase angles of remaining unit power cells An, Bn and Cn (nis positive integer) may be determined by the following equations usingthe displacement (S3). For convenience sake, a phase angle of the unitpower cell An is called An, and the same theory may be applicable to Bnand Cn.An=A1+(n−1)Δα_(sec)Bn=B1+(n−1)Δα_(sec)Cn=C1+(n−1)Δα_(sec)  [Equation 3]

At this time, when an each absolute value of phase angle of An, Bn andCn exceeds 30°, for example (S4), for example, the phase angle isadjusted (S5). That is, the phase angle may be adjusted as the followingequation.

$\begin{matrix}{{X^{\prime}n} = {{{sgn}({Xn})}\frac{{6{{Xn}}} - 360}{6}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

At this time, X is one of A, B and C. Furthermore, sgn(Xn) is a functiondetermining a sign, where when

$\frac{{6{{Xn}}} - 360}{6}$is positive, ‘1’ is outputted and when

$\frac{{6{{Xn}}} - 360}{6}$is negative ‘−1’ is outputted.

That is, when an absolute value of phase angle of An, Bn and Cn exceeds30° at the maximum (S4), the phase angle is adjusted (S5), and theadjusted phase angle is outputted as a final phase angle, and when anabsolute value of phase angle of An, Bn and Cn does not exceed 30° atthe maximum, the phase angle determined at S3 may be outputted as afinal phase (S6).

The phase angle determined by the present disclosure in a medium voltageinverter as in FIG. 5 may be such that A1 is 0°, A2 10°, B1 is 20°, B2is 30°, C1 is −20° and C2 is −10°.

Furthermore, when the number of unit power cells connected for eachphase is 3 as in FIG. 1, A1 may be 0°, A2 may be 6.7°, A3 may be 13.4°,B1 may be 20°, B2 may be 26.7°, B3 may be −26.7°, C1 may be −20°, C2 maybe −13.4°, and C3 may be −6.7°.

The THD of a total input phase current may be improved by applying thephase angle determined by the method according to the presentdisclosure, which is discussed as below:

Referring to FIG. 1 again, output voltages from the phase shifttransformer (110) to power cells A1, B1 and C1 (120 a, 120 d, 120 g)have the same phases, and the output voltages to the power cells A2, B2and C2 (120 b, 120 e, 120 h) have the same phases, and the outputvoltages to the power cells A3, B3 and C3 (120 c, 120 f, 120 i) alsohave the same phases.

In this case, the ‘a’ phase current at the primary side of the phaseshift transformer (110) is greatest at the 17th harmonic, and ‘b’ and‘c’ currents also have the greatest at the 17th harmonic. Now, themedium voltage inverter applied with the present disclosure will beexplained.

For example, ‘a’ phase current flowing in each of the power cells of A1,A2, B1, B2, C1, C1 in FIG. 5 may be defined by the following equations5-10.

$\begin{matrix}{i_{{aa}\; 1} = {\sum\limits_{{n = 1},5,7,11,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {n\;\omega\; t} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack \\{i_{{aa}\; 2} = {\sum\limits_{{n = 1},5,7,11,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {n\left( {{\omega\; t} + \delta_{1}} \right)} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack \\{i_{{ab}\; 1} = {\sum\limits_{{n = 1},5,7,11,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{2}} \right)} - {\frac{2}{3}\pi}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack \\{i_{{ab}\; 2} = {\sum\limits_{{n = 1},5,7,11,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{3}} \right)} - {\frac{2}{3}\pi}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack \\{i_{{ac}\; 1} = {\sum\limits_{{n = 1},5,7,11,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{4}} \right)} + {\frac{2}{3}\pi}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack \\{i_{{ac}\; 2} = {\sum\limits_{{n = 1},5,7,11,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{5}} \right)} + {\frac{2}{3}\pi}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

If a turn ratio between a primary side and a secondary side is assumedas 1:m, a current flowing to the primary side of the phase shifttransformer (11) may be defined by the following equations 11˜16.

$\begin{matrix}{i_{{aa}\; 1}^{\prime} = {\frac{1}{m}\left( {{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {n\;\omega\; t} \right)}}} + {\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {n\;\omega\; t} \right)}}}} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack \\\begin{matrix}{\mspace{79mu}{i_{{aa}\; 2}^{\prime} = {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}\sin\left( {{n\left( {{\omega\; t} + \delta_{1}} \right)} - \delta_{1}} \right)}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{1}} \right)} + \delta_{1}} \right)}}}\end{pmatrix}}}} \\{= {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}\sin\left( {{n\;\omega\; t} + {\left( {n - 1} \right)\delta_{1}}} \right)}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n + 1} \right)\delta_{1}}} \right)}}}\end{pmatrix}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack \\\begin{matrix}{\mspace{79mu}{i_{{ab}\; 1}^{\prime} = {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{2}} \right)} - \delta_{2}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{2}} \right)} + \delta_{2}} \right)}}}\end{pmatrix}}}} \\{= {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n - 1} \right)\delta_{2}}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n + 1} \right)\delta_{2}}} \right)}}}\end{pmatrix}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 13} \right\rbrack \\\begin{matrix}{\mspace{79mu}{i_{{ab}\; 2}^{\prime} = {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{3}} \right)} - \delta_{3}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{3}} \right)} + \delta_{3}} \right)}}}\end{pmatrix}}}} \\{= {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}\sin\left( {{n\;\omega\; t} + {\left( {n - 1} \right)\delta_{3}}} \right)}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n + 1} \right)\delta_{3}}} \right)}}}\end{pmatrix}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 14} \right\rbrack \\\begin{matrix}{\mspace{79mu}{i_{{ac}\; 1}^{\prime} = {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{4}} \right)} - \delta_{4}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{4}} \right)} + \delta_{4}} \right)}}}\end{pmatrix}}}} \\{= {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n - 1} \right)\delta_{4}}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n + 1} \right)\delta_{4}}} \right)}}}\end{pmatrix}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 15} \right\rbrack \\\begin{matrix}{\mspace{79mu}{i_{{ac}\; 2}^{\prime} = {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{5}} \right)} - \delta_{5}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\left( {{\omega\; t} + \delta_{5}} \right)} + \delta_{5}} \right)}}}\end{pmatrix}}}} \\{= {\frac{1}{m}\begin{pmatrix}{{\sum\limits_{{n = 1},7,{13\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n - 1} \right)\delta_{5}}} \right)}}} +} \\{\sum\limits_{{n = 5},11,{17\mspace{14mu}\ldots}}^{\infty}{I_{n}{\sin\left( {{n\;\omega\; t} + {\left( {n + 1} \right)\delta_{5}}} \right)}}}\end{pmatrix}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 16} \right\rbrack\end{matrix}$

Whereby ‘a’ current may be defined by the following equation 17, where35th harmonic is the greatest except for the fundamental wave.i _(a) =i _(aa1) ′+i _(aa2) ′+i _(ab1) ′+i _(ab2) ′+i _(ac1) ′+i _(ac2)′

In general, as an order of harmonics increases, the size of theharmonics decreases, such that although the 17th harmonic is thegreatest harmonic in the conventional inverter, the 35th harmonic is thegreatest in the present disclosure, whereby it can be noted that the THDis improved.

As apparent from the foregoing, the THD of an input phase current can bemitigated according to the present disclosure, because a phase angle ofa phase shift transformer in a medium voltage inverter is designed inconsideration of a total number of unit power cells.

Although the present disclosure has been described in detail withreference to the foregoing embodiments and advantages, manyalternatives, modifications, and variations will be apparent to thoseskilled in the art within the metes and bounds of the claims. Therefore,it should be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within thescope as defined in the appended claims

What is claimed is:
 1. A method for determining phase angle in phaseshift transformer for medium voltage inverter, the inverter including aplurality of unit power cells formed in first and second stages, onestage formed with 3-phase unit power cells, and the plurality of unitpower cells respectively connected to the phase shift transformer, themethod comprising: selecting an arbitrary phase shift angle and phaseangle relative to a unit power cell of the first stage; determining aphase angle displacement in consideration of the phase shift angle andthe number of unit power cells connected to each phase of a motor;determining a phase angle of the unit power cell at the second stageusing a phase angle of the unit power cell at the first stage; andadjusting a phase angle when the determined phase angle of the unitpower cell at the second stage exceeds a predetermined phase angle. 2.The method of claim 1, wherein the predetermined phase angle is 30° atthe maximum.
 3. The method of claim 1, wherein the adjustment of thephase angle is performed by using the following Equation$,{{X^{\prime}n} = {{{sgn}({Xn})}\frac{{6{{Xn}}} - 360}{6}}},$ whereX′n is an adjusted phase angle of the unit power cell, Xn is a phaseangle of the unit power cell (n is positive integer), and sgn(Xn) is afunction determining a sign, where when $\frac{{6{{Xn}}} - 360}{6}$ ispositive, ‘1’ is outputted and when $\frac{{6{{Xn}}} - 360}{6}$ isnegative ‘−1’ is outputted.
 4. The method of claim 1, wherein the phaseangle displacement is a value in which the phase shift angle of the unitpower cells in the first stage is divided by the number of unit powercells connected to each phase of the motor.
 5. The method of claim 1,wherein the phase angle of the plurality of unit power cells isindividually determined relative to each unit power cell.